In state of the art torque converters, a torque converter lockup clutch is disposed, which is engaged when small rotation speed differences between the pump and the turbine are present, in order to improve the efficiency of the torque converter. When the torque converter lockup clutch is actuated through oil pressure, a friction locked connection between the torque converter housing and the hub, into which the transmission input shaft of the subsequent transmission engages, is established. In the force flow between the torque converter housing and the hub, a torsion oscillation damper can be located in addition.
The piston of the torque converter lockup clutch has a bore in its center, through which it can be concentrically located on a hub. This hub can be a turbine hub, as well as another hub. It is only important in this context, that the piston can be moved in an axial direction as easily as possible, so that the friction moment between the piston and the torque converter housing can be adjusted as sensitively as possible. In order to move the piston in an axial direction, a differential oil pressure is built up on both sides of the piston, so that an axial force component is generated. A pressure loading is most common, in which a higher pressure prevails in the section between the piston and the torque converter turbine, than in the section between the piston and the engine side torque converter housing.
Through the axial movability of the piston on the hub, an oil leakage flow is possible here, therefore, according to the state of the art, a seal is located between the bore of the piston and the hub associated with it. Thus optimum sealing at this location is difficult, since, on the one hand, an axial movability of the piston has to be assured, which is as smooth as possible, and, on the other hand, the leakage oil flow has to be kept as small as possible. In addition, there is the problem that different speeds of rotation between the piston and the hub exist, depending on the operating situation of the torque converter, through which the relative rotational movement between the piston and the hub are present. Only when the torque converter lockup clutch is completely locked, no relative rotational movements occur.
When accelerating from a stop, the relative speeds of rotation between the piston and the hub are in the range of 600 to 2,500 RPM; in the general driving range the relative speeds of rotation are still in the range of 10 to 60 RPM. This means, that the seal also has to be durable under relative rotation besides its primary sealing function, since this component cannot be replaced due to the enclosed shape of the torque converter.
In order to assure a comfortable control of the torque converter lockup clutch under low torque, in particular during coasting, it is necessary that the seal already seals under very low differential pressures on both sides of the piston (<0.1 bar). If this is not the case, a leakage occurs at the gasket, which is spontaneously reduced in an excursive, thus unpredictable, manner, when the pressure increases further. Due to the substantially constant characteristic pump curve of the oil feed pump, the spontaneous sealing between the piston and the hub causes a step in the differential pressure between the two outside piston surfaces, whereby also a spontaneous stronger pressing of the piston is performed, whereby in turn an increased friction moment is generated. This step in the friction moment leads to a jolt, which is perceived by the driver of the vehicle as uncomfortable.